As signal recording media for optical disc players, two kinds of discs are generally available: CAV (constant angular velocity) discs wherein video signals are recorded at a constant angular velocity for two fields per turn of track, and CLV (constant linear velocity) discs wherein video signals are recorded at a constant linear velocity.
With the CLV disc, synchronizing signals are recorded on a spiral track at equal spacings and are not aligned radially of the disc, so that when the pickup is caused to jump the track radially of the disc for special reproduction such as still-picture reproduction, the period of reproduction synchronizing signals is disturbed. Consequently, a color loss, disturbance in vertical synchronization, horizontal skew or the like occurs in the picture that is reproduced.
Accordingly, a video disc player has been proposed which is adapted to reproduce a still picture from the CLV disc by repeatedly causing the pickup to jump from a first position to a second position preceding the first position by approximately one frame while detecting these first and second positions so as to intermittently output the signals of one frame recorded in the track portion of the second position through the first position (Unexamined Japanese Patent Publication SHO 58-139577).
However, the proposed video disc player requires a complex circuit for detecting the first and second positions. Moreover, the jumping action of the pickup for still-picture reproduction makes discontinuous the color subcarrier which is prepared based on the color burst extracted from the reproduction video signal, giving rise to the problem that a color loss occurs in the picture reproduced.
With advances in video memory techniques in recent years, on the other hand, inexpensive field memories have been developed which are capable of storing video signals for one frame. The use of such field memories in video disc players has been investigated to realize various modes of special reproduction.
Nevertheless, with field memory application techniques heretofore generally known, the period of writing video signals in the field memory and the period of reading video signals therefrom are both accurately set to 262.5 times the regular field period (i.e. horizontal synchronization period Th) so that the following problem is encountered.
When video signals of NTSC system are written in the field memory with the period of 262.5Th and are thereafter repeatedly read out from the momory with the period of 262.5Th for reproducing a still picture, the writing/reading period includes the fraction of 0.5Th thereby disturbing at the junction of fields the period of horizontal synchronizing signals included in the reproduction signals to create a skew.
With the NTSC system, there is the following relationship between the color subcarrier period Tc and the horizontal synchronizing signal period Th,
wherein EQU Th=(455/2)Tc.
Accordingly, the number of waves of the color subcarrier included in one field period (262.5Th) is not an integer with the result that the subcarrier period also involves a disturbance at the junction of fields which gives rise to the problem of a color loss.
On the other hand, the magazine "TV Technology" (published by Nippon Denshi Shuppan Co., Ltd., Nov. 1985, pp. 35-39) discloses a video tape recorder wherein the period of reading out data from the field memory is changed over between 262Th and 263Th for every field, such that the change-over period is set to 525Th (=262.5Th.times.2) for every two fields; i.e., every frame. This method, if applied to the video disc player, obviates the disturbance in the period of horizontal synchronizing signals at the field junction to preclude the occurrence of the skew.
Neverthless, when the reading period for the field memory is set to 263Th, the period 263Th has the following relationship with the period Tc: EQU 263Th=263.times.(455/2)Tc=59832.5Tc.
The number of waves of the color subcarrier included in this period is therefore not an integer so that the color subcarrier period becomes inevitably disturbed at the field junction.